1. Field of the Invention
This invention relates to a synthetic quartz powder having few residual gases and a low carbon and hydroxyl group content, its production process, and a quartz crucible having a low bubble content, which is made with the synthetic quartz powder.
2. Discussion of the Background
A sol-gel process is known in which synthetic quartz powder is made by drying and baking a gel obtained by hydrolyzing a metal alkoxide. In this process an ethyl-silicate is hydrolyzed to be gelled (nSi(OH)4), the gelled ethyl-silicate is pulverized and dried to obtain a silica gel powder, and the silica-gel powder is baked at a predetermined temperature (about 1050xc2x0 C.) and dehydrated to obtain an amorphous silica powder (nSiO2).
It is also known that the synthetic quartz powder made by this process is of high purity, having fewer metal impurities than a natural quartz powder, but that carbon and the hydroxyl group causing the alkoxy group remain. From this reason, when a quartz glass crucible is made by using the synthetic quartz powder obtained by the sol-gel method as a raw material, the carbon may cause bubbles to form.
Therefore, in the process for producing the synthetic quartz powder by the sol-gel method, a trial is done in which residual carbon is combusted and removed before the synthetic quartz powder is made by baking a dry gel powder. Processes are well known in which the amount of residual carbon is reduced by heat-treating the dry gel powder, where the carbon is changed to carbon gases by combustion at a temperature of less than 600xc2x0 C., at which pores of the dry gel powder are not closed, in an oxygen atmosphere (generally in air). (Japanese Patent Laid Open No. Hei 09-86916 and Japanese Patent Application No. Hei 10-287416, etc.) In all of the processes, the residual carbon in the dry gel powder is combusted and removed before the pores of dry gel powder are closed. That is, an amorphous synthetic quartz powder is made by baking the dry gel powder at about 1000xc2x0 C. to 1300xc2x0 C., where the dry gel powder is vitrified (closing pores), after decarbonizing at less than 600xc2x0 C.
Moreover, it is also known that the process, in which the amorphous synthetic quartz powder is made by pulverizing a quartz glass ingot, which is made by baking the silica powder made by the sol-gel method after heat-treating. (Japanese Patent Publication of Examined Application No. Hei 05-63416) The process removes an OH group by fusing the silica powder made by the sol-gel method, which is heated at 1500xc2x0 C. after combusting organic materials at about 500xc2x0 C. in air. Moreover, the process is common in the former process, wherein the residual carbon is combusted to be removed, by heating in air at a temperature of less than 600xc2x0 C., at which the pores of dry gel powder are not closed. In addition, in the process, the heating is done in a low pressure atmosphere at time of making the glass ingot at about 1500xc2x0 C. for evacuation removal of the air contained in the silica powder, and the removal of the residual carbon is mainly done in the oxidization combusting process before vitrifying at less than 600xc2x0 C. In the high temperature sintering at about 1500xc2x0 C., it is difficult to remove the carbon since the silica powder is sintered.
Furthermore, a process is also known (Japanese Patent Registration No.2530225) in which a predetermined high-density synthetic quartz powder is made by baking in two stages under a dry atmosphere or a reduced pressure, where the synthetic quartz powder is made by baking dry gel powder. Although the process shows baking under a reduced pressure as one mode of the baking process, since the process has the objective of increasing the effect of dehydration and the vacuum degree is low, the process is almost ineffective for decarbonization. That is, since this baking process heats with stirring while avoiding heating in the reduced pressure container, it is difficult to keep high vacuum and the limit of the reduced pressure is about 0.5 atmosphere in a real operation. Therefore, the residual carbon cannot be removed at such a reduced pressure.
About the synthetic silica powder made by the sol-gel process in this way, although it is conventionally known that the process, in which the residual carbon contained in silica powder is combusted in air to be removed, the limit of concentration of the residual carbon in the silica powder by said process is about 5 to 100 ppm, and it is difficult to reduce the residual carbon to less than this value.
The present invention solves the above-mentioned problems, and provides a treatment process that reduces the residual gases of the synthetic quartz powder made by a wet process, especially the amount of the residual carbon and the hydroxyl group content. Moreover, the present invention provides a synthetic quartz powder made by the treatment process, and a quartz glass crucible having few bubbles contents made by the synthetic quartz powder.
That is, this invention relates to a process for producing of synthetic quartz powder in the following embodiments.
(1) A process for producing of a synthetic quartz powder, wherein a silica gel powder or a synthetic quartz powder made by wet process is baked under a low pressure atmosphere of less than 100 Pa and at a temperature being from more than decarbonizing to less than powder sintering.
(2) A process for producing of a synthetic quartz powder, wherein a silica gel powder made by wet process is baked to be a synthetic quartz powder under an air atmosphere and at a temperature from more than removing a hydroxyl group to less than powder sintering, and the synthetic quartz powder is baked under a low pressure of less than 100 Pa and at a temperature being from more than decarbonizing to less than powder sintering.
(3) The process for producing a synthetic quartz powder according to above-mentioned (2), wherein the baking atmosphere is a dry air or an oxidizing atmosphere, the baking temperature is more than 800xc2x0 C. to less than 1400xc2x0 C., and a baking time is 5 to 70 hours.
(4) The process for producing a synthetic quartz powder according to above-mentioned (1) to (2), wherein the low pressure is less than 50 Pa and the baking temperature is more than 600xc2x0 C. to less than 1400xc2x0 C.
(5) The process for producing a synthetic quartz powder according to above-mentioned (1) to (2), wherein the baking time in the low pressure atmosphere is defined by the time required to reach a specified low pressure.
(6) The process for producing a synthetic quartz powder according to above-mentioned (5), wherein the baking in the low pressure atmosphere is finished when the pressure decreases to less than 5 Pa.
Moreover, this invention relates to a following synthetic quartz powder and quartz glass crucible.
(7) A synthetic quartz powder made by any processes according to above-mentioned (1) to (2), wherein a carbon content of the powder is less than 2 ppm and a hydroxyl group content is less than 50 ppm.
(8) A quartz glass crucible made by a synthetic quartz powder, wherein at least a part of an inside surface of the crucible is made using the synthetic quartz powder of the above-mentioned (7) as a raw material.
(9) A quartz glass crucible made by a synthetic quartz powder, wherein at least a part of the inside surface of the crucible is made using the synthetic quartz powder of the above-mentioned (7), and a transparent glass layer 0.5 mm thick forming the inside surface of the crucible has a bubble content of less than 0.1%.
The process for producing of this invention reduces residual gases in the powder, especially the residual carbon, by baking the silica gel powder made by a wet process, or the synthetic quartz powder made by baking the silica gel powder, while keeping the state of the powder under the following conditions. The reduced pressure is lower than a medium vacuum, that is, the reduced pressure is less than 100 Pa, and is preferably to less than 50 Pa. The temperature is from more than a decarbonizing temperature to less than a powder sintering temperature, that is, specifically, for example, more than 600xc2x0 C. to less than 1400xc2x0 C. In addition, according to the process of this invention, an amorphous synthetic quartz powder having a residual carbon content of less than 2 ppm can be produced.
In addition, the process for producing of this invention also includes the process that bakes to decarbonize in the above-mentioned vacuum, after baking to remove the hydroxyl group in an air atmosphere while keeping the powder state. By baking in two stages in an air atmosphere and a low pressure atmosphere at the specific temperature range, the amorphous synthetic quartz powder, which is remarkably reduced in residual hydroxyl groups and residual carbon, is made. Specifically, in the above-mentioned synthetic quartz powder, the residual carbon is less than 2 ppm, and the residual hydroxyl group is less than 50 ppm.